Optical members used in liquid crystal displays, for example a polarizing plate and a retardation plate, are attached via a pressure sensitive adhesive to liquid crystal cells. Materials used in such optical members expand and contract significantly under heating conditions and moistening conditions so that under such conditions, dry spots and lifting easily occur after attachment. Accordingly, the pressure sensitive adhesive for optical members is required to be durable even under heating conditions and moistening conditions.
When an attachment surface upon attaching an optical member is bitten with foreign matter, or an optical member is out of position because of erroneous attachment, the optical member is released for reutilization from a liquid crystal cell. When the optical member is released from the crystal cell, the re-releasability of the pressure sensitive adhesive to easily release the optical member is required so as to prevent a gap of the liquid crystal cell from being changed or the liquid crystal cell from being broken. However, when techniques of merely improving the adhesion state of the optical member are adopted for the reason the durability of the pressure sensitive adhesive for optical members is regarded as important, the pressure sensitive adhesive is rendered inferior in re-releasability.
Further, the pressure sensitive adhesive for optical member is required to uniformly relax stress caused by a dimensional change in optical members such as a polarizing plate under heating or moistening conditions. When the pressure sensitive adhesive is inferior in relaxation of stress, residual stress remains in the optical members such as a polarizing plate, to bring about adverse influences such as discoloration and coloring failure in some cases.
Various kinds of materials are proposed as the pressure sensitive adhesive used in optical members. For example, it is attempted to improve the stress relaxation of the pressure sensitive adhesive by blending a low-molecular weight polymer with a high-molecular weight polymer (see JP-A 10-279907, JP-A 2000-109771, JP-A 2000-89731, JP-A 2001-335767, and JP-A 2002-121521).
JP-A 10-279907 proposes a pressure sensitive adhesive composition having a crosslinked structure formed by blending 20 to 200 parts by weight of a low-molecular weight polymer having a weight-average molecular weight of 30,000 or less with 100 parts by weight of a high-molecular weight polymer having a high functional group proportion. It is disclosed therein that a three-dimensional structure of the high-molecular weight component in the pressure sensitive adhesive composition prevents foaming and peeling under high temperature and high humidity, and internal stress caused by a dimensional change in a polarizing plate can be absorbed by the low-molecular weight polymer component.
JP-A 2000-109771 proposes a pressure sensitive adhesive composition comprising a high-molecular weight polymer blended with a low-molecular weight polymer having a weight-average molecular weight of 500,000 or less. It is disclosed therein that the pressure sensitive adhesive composition can relax stress concentration to prevent coloring failure and discoloration of a liquid crystal cell, and after released, does not permit any residual adhesive or fogging on the liquid crystal cell.
JP-A 2000-89731 proposes a pressure sensitive adhesive composition comprising 1 to 50 parts by weight of a low-molecular weight polymer having a weight-average molecular weight of not less than 5,000 and less than 500,000 blended with 100 parts by weight of a high-molecular weight polymer, wherein either of the high-molecular weight polymer or the low-molecular weight polymer contains nitrogen-containing functional groups. It is disclosed therein that the pressure sensitive adhesive composition is excellent in durability due to binding of the nitrogen-containing functional groups to an adherent, and prevents coloring failure by coping with expansion and contraction of a polarizing plate.
JP-A 2001-335767 proposes a pressure sensitive adhesive composition comprising 5 to 100 parts by weight of an acrylic oligomer having a weight-average molecular weight of 1,000 to 10,000 and a bifunctional crosslinking agent blended with 100 parts by weight of a high-molecular weight polymer. It is disclosed therein that the pressure sensitive adhesive composition is excellent in adhesion to an adherent and possesses excellent relaxation of stress, to attain durability and prevent coloring failure.
JP-A 2002-121521 proposes a pressure sensitive adhesive composition comprising 10 to 100 parts by weight of a low-molecular weight polymer having a glass transition point of 0 to −80° C. and a weight-average molecular weight of 30,000 to 100,000 and a multifunctional compound blended with 100 parts by weight of a high-molecular weight polymer. It is disclosed therein that the pressure sensitive adhesive composition can cope with peeling, foaming and coloring failure phenomenon, and is excellent in reworkability (re-releasability) concerned with the degree of difficulty in releasing a polarizing film from a liquid crystal cell and in re-attaching it.
Any of the above pressure sensitive adhesive compositions comprising a low-molecular weight polymer blended with a high-molecular weight polymer are excellent in durability and absorb internal stress by the low-molecular weight component. However, it cannot be said that any of the above pressure sensitive adhesive compositions sufficiently satisfy durability, re-releasability and stress relaxation. Particularly, the pressure sensitive adhesive compositions are poor in the ability to re-release optical members in order to re-utilize liquid crystal cells. In evaluation of reworkability (re-releasability) in JP-A 2002-121521, reworkability are assumed to be excellent when a pressure sensitive adhesive polarizing film previously attached to a glass plate, treated in an autoclave and left at 23° C. under 65% RH for 24 hours shows an adhesion of not higher than 1200 g/25 mm (about 12 N/25 mm) upon peeling at 180°. According to this standard of reworkability, the reworkability of a larger crystal cell as an evaluation sample (sample width: 250 mm) are assumed to be excellent when the adhesion thereof upon peeling is not higher than 12 kg/250 mm. Under this standard, however, the gap of such a larger liquid crystal cell is often broken.
On the other hand, liquid crystal displays come to be used in personal computers and TV sets, and large liquid crystal displays are recently usable, and the number of large liquid crystal displays is rapidly increasing. At present, waste materials from the liquid crystal displays are not so much, but it is estimated that the waste materials are rapidly increased in near future, and the amount of the waste materials is estimated to reach about 100,000 tons annually. Waste materials from TFT liquid crystal panels, for example, are composed of 85 wt % glass and 15 wt % polarizing film (resin). The glass is milled and used as e.g. glass cullets for building materials, but the amount of such reused glass is not so great at present. This is because isolation of glass is difficult. Accordingly, there is demand for techniques of separating glass from other components. Methods of recycling used liquid crystal displays are disclosed (see JP-A 2000-24613, JP-A 2000-189939 and JP-A 2000-84531), but these methods involve heating the liquid crystal displays at high temperatures where enormous energy is required, thus bringing about a problem of higher costs. In addition, methods of fractionating and recovering materials constituting liquid crystal displays to recycle them are also disclosed (JP-A 2002-159955 and JP-A 2001-328849). However, methods using special devices or leading to generation of waste alkali solutions are hardly generally utilizable because of problems in costs and waste liquor treatment. An anti-reflection film provided with an adhesive, which upon disposal, is readily releasable is disclosed (JP-A 11-209708).
An object of the present invention is to provide a pressure sensitive adhesive composition for optical members, which is excellent in durability, re-releasability and stress relaxation. Another object of the present invention is to provide a pressure sensitive adhesive composition for optical members, which hardly increases adhesion after storage for a long time and does not leave a residual adhesive on glass upon disposal or repair. A further object of the present invention is to provide a pressure sensitive adhesive layer for optical members, which is formed from the pressure sensitive adhesive composition for optical members. A still further object of the present invention is to provide a pressure sensitive adhesive optical member having the pressure sensitive adhesive layer, as well as an image display using the pressure sensitive adhesive optical member.